Try to make a device to hold the meter and consistently place it in the same area, and use the same placement of the organization you're building for. Also doesnt hurt to do testing in other areas of the vehicle too. Sealed on the dash location is a good standard to use- iirc it's 12 inches over from the a pillar and 5 inches from the base of the windsheild.
When youre testing you can try this for gains: open glovebox, close glovebox, move seats fore and aft, tilt seatbacks, move headrests, move sunvisors, windows up and down, different doors open and closed, drivers side vs pass side, move box around in the back, with or without a person in the seats etc. Some of those might gain a few tenths, some might loose tenths, you never know.